This application is a continuation-in-part application of U.S. patent application, Ser. No. 08/815,273, filed Mar. 10, 1997. The present invention relates to an exposure apparatus and method, and more specifically, to a lithography process and apparatus used for the fabrication of items such as ICs, liquid crystal substrates, thin-film magnetic heads, etc.
Prior exposure systems for the fabrication of ICs and liquid crystal substrates employed various mechanisms to correct changes in the image-forming condition (e.g., magnification or focal position) due to factors such as changes in atmospheric pressure or absorption of exposure light by a projecting optical system. Methods proposed for these correction mechanisms in the past include, for example, shifting individual lens elements forming a projecting optical system along the optical axis, tilting them with respect to planes perpendicular to the optical axis, and adjusting the pressure in the airtight spaces between lens elements (see U.S. Pat. No. 4,666,273).
However, since the exposure light passes through the mask, some of it is absorbed, which heats the mask, causing thermal expansion. This creates problems in that it changes the image-forming condition of the projecting optical system (i.e., the location and magnification of a projected image of the mask pattern). Technology to solve this problem was proposed in Japanese Kokai (Laid-Open Patent Application) No. H4(1992)-192317. This solution involved calculating the amount of thermal deformation of a mask caused by absorption of an irradiated light using a prescribed numerical calculation. After calculating a change in the image-forming properties caused in response to this amount of thermal deformation , the lens elements of the projecting optical system are driven in the optical axis direction or an inclined direction around an axis at right angles to this optical axis, on the basis of the result of this calculation, thereby obtaining a desired image-forming condition.
The thermal expansion of the mask causes a change in distance between the position of the projected image of the mask pattern and the detecting position of a detecting optical system detecting a mark on a wafer serving as the sensitive substrate (baseline). This creates an additional problem in that it degrades the overall accuracy of the overlay alignment when multiple additional mask pattern images are superimposed over mask patterns formed on a wafer. This also could cause alignment error between the mask and the wafer in an operation known as global alignment.
For the problem of change in the baseline, on the other hand, the amount of thermal expansion of the mask in a two-dimensional plane at right angles to the optical axis caused by thermal expansion of the mask is determined by calculation. A positional deviation of the wafer relative to the mask exposure apparatus is avoided by controlling the amount of baseline from time to time on the basis of the result of this calculation when the amount of thermal expansion of the mask increases above a prescribed value (Japanese Unexamined Patent Publication No. 7-74075).
However, while the foregoing technique disclosed in Japanese Unexamined Patent Publication No. 4-192317 permits correction of a change in the image-forming condition caused by thermal expansion of the mask, it is impossible to correct a positional deviation of the mask relative to the exposure apparatus. An image of the pattern region of the mask is formed on a shot region on the wafer serving as the exposed substrate with a positional deviation or a rotational deviation.
In the conventional art of controlling from time to time the baseline (hereinafter appropriately referred to as the xe2x80x9cbaseline checkingxe2x80x9d) when the calculated amount of thermal expansion of the mask is above a prescribed value, as disclosed in Japanese Unexamined Patent Publication No. 7-74075, it is necessary to frequently perform baseline checking, which takes from several seconds to several minutes as long as the mask continues to thermally expand. This leads to a considerable deterioration of throughput of the exposure apparatus.
The present invention was developed in view of the foregoing problems, and has as an object to provide an exposure apparatus and a method of exposure which do not cause a deterioration of throughput of the exposure apparatus and do not cause a change in baseline or a change in magnification resulting from thermal expansion of the mask.
Accordingly, it is an object of the present invention to overcome the problems involved with prior art solutions. The present invention was devised with the above problems in mind. Its objective is to provide an exposure method and apparatus through which the detrimental effects of thermal expansion of the reticle can be easily prevented, with no need to perform complex and involved computations.
In order to achieve the above objective, the present invention performs alignment checks and exposure using a mask that has already reached the thermal expansion saturation state, not accounting for mask thermal distortion (thermal expansion distortion, for example) as was necessary in the past. In the following description, thermal expansion is used as an example of mask thermal distortion. Accordingly, since the mask is in a state of thermal expansion saturation, it neither expands nor contracts, holding its shape constant. This method solves the above problems due to mask thermal expansion. The present invention, which is based on this principle as the solution to these problems, is comprised as described below.
A method incorporating the principles of the present invention includes a method for exposing an image of a pattern formed in a mask onto a sensitive substrate. It includes the steps of irradiating the mask with an exposure light and computing data related to the thermal expansion saturation point of the mask due to absorption of the irradiation light. It also includes expanding the mask to the thermal expansion saturation point based on the data computed in the computing step. Finally, the image of the mask pattern is exposed onto the sensitive substrate.
The exposure method incorporating the principles of the present invention can also include an additional step, performed after the mask-expanding step and before the step of exposing the sensitive substrate, of correcting the magnification of the mask pattern.
The method incorporating the principles of the present invention may also comprise a further step, performed after the mask-expanding step and before the step of exposing the sensitive substrate, of aligning the relative positions of the mask and the sensitive substrate such that the center of the mask image is aligned with the center of a shot area of the sensitive substrate.
To achieve the above objective in another mode of the present invention, in an exposure method for exposing an image of a pattern formed in a mask onto a sensitive substrate, the invention comprises the steps of determining the change in the thermal expansion state of the mask due to absorption of exposure light for a predetermined time period, determining the temperature at which thermal expansion saturation of the mask occurs based on the change in the thermal expansion state thereof, and heating the mask to the determined temperature. The final step is exposing the image of a pattern in the mask onto the sensitive substrate.
The method may also include an additional step of controlling the temperature of the mask to hold the temperature of the heated mask at the thermal expansion saturation temperature.
To achieve the above objective, the apparatus according to the principles of the present invention provides an apparatus for exposing an image of a pattern formed in a mask onto a sensitive substrate, and includes an exposure source for irradiating the mask with an irradiation light. It also includes a computation section for computing data related to the thermal expansion saturation point of the mask due to absorption of irradiation light. A memory is provided for storing the data computed by the computation section. Finally, an expansion system is included for expanding the mask based on the data stored in the memory, before starting exposure of the sensitive substrate.
It is desirable for the above expansion system to expand the mask by irradiating it with an exposure light.
It is also desirable for the above expansion system to have a heater for heating the mask.
In addition, the exposure apparatus incorporating the principles of the present invention can also include a control section for controlling the passing and blocking of irradiation. The computation section can compute its data based on the irradiation passing and blocking time periods as controlled by the control section.
In addition, the exposure apparatus incorporating the principles of the present invention can comprise a correction system for correcting the magnification of the mask pattern image. The correction system will change the length of the optical path between the mask and the sensitive substrate in the state wherein the mask has reached the thermal expansion saturation point.
Furthermore, in the computation section of the exposure apparatus incorporating the principles of the present invention, it is desirable for the thermal expansion saturation point of the mask to be computed based on at least one value from among the values determined in accordance with the exposure light, and the values determined in accordance with the characteristics of the mask.
In addition, in the exposure apparatus, it may be desirable for those values determined in accordance with the exposure light to be the luminous energy of the exposure light and the irradiated area on the mask, and for those values determined in accordance with the characteristics of the mask to be the transmissivity of the mask, the reflectivity of the mask, the material of the mask, and the density distribution of the pattern of the mask.
A further preferred embodiment of the present invention includes an exposure apparatus for irradiating an exposure light within a prescribed wavelength range onto a mask to expose a pattern on the mask onto a sensitive substrate. Storage means are included for storing the mask, as well as transfer means for transferring the mask between the storage means and an exposure position. Finally heating means are included for heating the mask in the storage means.
The present invention is also directed to a method of exposure of irradiating an exposure light within a prescribed wavelength range onto a mask, and copying a pattern of the mask onto a substrate. Information of temperature at which thermal expansion of the mask reaches saturation is calculated, and prior to irradiation of the exposure light onto the mask, the substrate is heated on the basis of the information thus calculated.